Look, the truth is that your friend is never going to famous author. So best case scenario is he listens to you, makes his book more readable and... no one is gonna buy it because he'll need to spend a few hundred on advertising to make any sales. The worst case scenario is that he's resentful about you "stifling his creativity" or becomes too self-conscious to publish, and the result is still the same, no one buys his book.

So you're risking your friendship for something that won't make any difference in book sales.

A lot of writers are just objectively bad, but the process of writing and designing a book makes them happy, and as their friends, that's what we should support.